Digital television has a cliff-effect where either the entire service is received or none of the service is received. This is the effect of a forward error correction (FEC) operation used in digital demodulation. Old analog televisions had picture quality depend on received RF signal strength. Digital television currently cannot offer this service of graceful degradation in picture quality. The concept set forth herein returns this graceful performance degradation to digital television.
Quadrature amplitude modulation (QAM) is a technique which conveys data by changing some aspect of a carrier signal (symbol) in response to a data signal. In the case of QAM, the amplitude of two waves, ninety degrees out-of-phase with each other (in quadrature) are modulated or keyed to represent the data signal. In QAM, symbols representing N-bits of data are mapped to predetermined locations in a constellation of 2N points, with each point in the constellation being reserved for a respective symbol representing a particular binary sequence of data bits. With this understanding and the issue of graceful degradation of service mentioned above in mind, present principles are provided.